The mother of one of the two teenagers who murdered a dozen fellow students and a teacher in the massacre at Columbine high school has broken a decade of silence to say that she is unable to look at another child without thinking about the horror and suffering her son caused.

Susan Klebold, whose son Dylan and another youth, Eric Harris, hunted down pupils at the Colorado school with shotguns, a semi-automatic pistol and a rifle before killing themselves, has described her trauma over her son's actions.

"For the rest of my life, I will be haunted by the horror and anguish Dylan caused," she wrote in O, The Oprah Magazine. "I cannot look at a child in a grocery store or on the street without thinking about how my son's schoolmates spent the last moments of their lives. Dylan changed everything I believed about myself, about God, about family and about love."

Neither the Klebold nor Harris families has spoken about the massacre, in which 21 students were also wounded.

Klebold recounts how the last word she heard from her son was a gruff goodbye as he rushed out of the door early on the morning of the killings in April 1999.

"I was getting dressed for work when I heard Dylan bound down the stairs and open the front door … I poked my head out of the bedroom. 'Dyl?' All he said was 'Bye.' … His voice had sounded sharp. I figured he was mad because he'd had to get up early to give someone a lift to class. I had no idea that I had just heard his voice for the last time," she said.

Dylan Klebold was headed to make a final video with Harris to say goodbye and apologise to their families before they drove to the school to plant bombs, which failed to detonate, and to carry through their plan to kill their fellow students.

After the killings, the authorities said there were indications that the two youths were disturbed and hints of the looming catastrophe. Harris's blog included instructions on how to make explosives and, later, angry denunciations of society that attracted the attention of the police after Harris posted a death threat against another student. Closer to the massacre, Harris listed his stockpile of weapons and posted a hit list. Klebold was less overt but with Harris made secret videos of their weapons and wrote in his diary of a desire to plan an attack that would match the bombing in Oklahoma City by rightwing militiamen that killed 168 people.

Klebold writes that she had no idea that Dylan was contemplating killing himself or anyone else. "From the writings Dylan left behind, criminal psychologists have concluded that he was depressed and suicidal. I'd had no inkling of the battle Dylan was waging in his mind," she wrote.

"Dylan's participation in the massacre was impossible for me to accept until I began to connect it to his own death. Once I saw his journals, it was clear to me that Dylan entered the school with the intention of dying there. In order to understand what he might have been thinking, I started to learn all I could about suicide."

Five years after the killings, the FBI said they believe that Harris was a clinical psychopath who masterminded the plan and Klebold depressive.

The massacre continues to generate debate about the motives of the two youths and whether anything could have been done to stop them. The magazine said that Susan Klebold was not paid for the article and will not be making an appearance on Oprah Winfrey's television show.